I would trust the datasheet.
The correction can be applied after you make the measurement -- there is no need to update the sensor. For the record, you need to acquire both the ambient (chip) temperature Ta, and the object temperature Tom in Kelvin. The corrected object temperature Toc is given by the following expression in C, where eps is the actual surface emissivity:
Toc = pow( ( ( (pow(Tom,4)-pow(Ta,4))/eps) + pow(Ta,4) ), 0.25);